Setting link file permission in home-files-service-type

[c4droid]

Hi, Guix!

I'm changing my mailing workflow from fetchmail + procmail + msmtp +
mutt to fdm + msmtp + mutt, configuration file is already prepared, the
fdm and msmtp configuration file need 600 permission, guix home
home-files-service-type give 644 default, have workaround for when
symlinking change permission for target file.

My home.scm configuration cut:
``` scheme
(home-environment
 (services
  (list
   (service home-files-service-type
            `((".fdm.conf" ,(local-file "dotfiles/fdm.conf"))
              (".msmtprc" ,(local-file "dotfiles/msmtprc")))))))
```